The present invention relates to a surge protection apparatus for preventing electrostatic breakdown of an electronic function circuit.
FIG. 10 shows a conventional example of a surge protection apparatus. This is an equivalent circuit diagram of the surge protection apparatus disclosed in Japanese Laid-open Patent No. 58-159370.
In FIG. 10, plural (two, in this example) signal terminals 2 and 3, and a pair of power supply terminals 4 and 5 are connected to an internal circuit 1. Between the two signal terminals 2 and 3 and the negative (or zero volt) power supply terminal 5, diodes 6 and 7 for discharging the negative electric charge are connected in the direction shown. On the other hand, between the signal terminals 2 and 3 and the power supply terminal 5, a positive charge protective circuit 8 for discharging the positive electric charge is connected. The positive charge protective circuit 8 comprises diodes 9 and 10 connected to the signal terminals 2 and 3, and a transistor 11 those collector is connected to the cathodes of the diodes 9 and 10, and whose emitter is connected to the power supply terminal 5, and whose base is open.
The operation of the surge protection apparatus in FIG. 10 is as follows.
While the input voltage applied to the signal terminals 2 and 3 are within a range of operating voltage of the internal circuit 1, the transistor 11 is in its cut-off state, and the positive charge protective circuit 8 is at a high impedance. The diodes 6 and 7 are also in then cut-off state and are at a high impedance. Accordingly, the surge protection apparatus does not operate at all, and the voltages applied to the signal terminals 2 and 3 are directly supplied to the internal circuit 1, and ordinary signal processing is done.
When, due to some reason or other, negative high voltages are applied to the signal terminals 2 and 3, the diodes 6 and 7 are made to conduct. Accordingly, the negative high voltages are applied via bypass to the power supply terminal 5 through the diodes 6 and 7. As a result, breakdown of the internal circuit 1 due to negative high voltages will be prevented.
To the contrary, when positive high voltages exceeding the tolerance of the transistor 11 are applied to the signal terminals 2 and 3, the transistor 11 breaks down at BV.sub.CEO (the breakdown voltage between the collector and emitter with the base open). Consequently, the input voltages at the signal terminals 2 and 3 are clamped at BV.sub.CEO. Thus, by limiting the input voltages applied to the signal terminals 2 and 3 by the positive charge protective circuit 8, the internal circuit 1 may be protected from the breakdown due to positive static electricity.
FIG. 12 shows another example of a conventional surge protection apparatus. This is the surge protection apparatus disclosed in Japanese Patent Publication No. 48-30189.
In FIG. 12, a signal terminal 12 and a pair of power supply terminals 4 and 5 are connected to an internal circuit 1. Between the signal terminal 12 and the positive power supply terminal 4, a diode 13 for discharging the positive electric charge in the polarity shown is connected. On the other hand, between the signal terminal 12 and the negative (or zero volt) power supply terminal 5, a diode 14 for discharging the negative electric charge in the polarity shown is connected.
The operation of the surge protection apparatus in FIG. 12 is described below.
While the input voltage applied to the signal terminal 12 is within a range of supply voltage, the diodes 13 and 14 are both in their cut-off state and are at a high impedance. Accordingly, the surge protection apparatus does not operate at all, and the input voltage applied to the signal terminal 12 is directly supplied into the internal circuit 1, and ordinary signal processing is done.
On the other hand, due to some reason or other, if a positive high voltage exceeding the supply voltage is applied to the signal terminal 12, the diode 13 is made to conduct, and the input voltage applied to the signal terminal 12 is clamped. To the contrary, when a negative high voltage exceeding the supply voltage is applied to the signal terminal 12, the diode 14 is made to conduct, and the input voltage applied to the signal terminal 12 is clamped.
The surge protection apparatus shown in FIG. 12 protects the internal circuit 1 from breakdown due to surge voltage in this way.
The conventional surge protection apparatus shown in FIG. 10 and FIG. 12, however, have the following problems.
In the surge protection apparatus in FIG. 10, in actual use, that is, when a supply voltage is applied to a pair of power supply terminals 4 and 5 and input voltages not exceeding the supply voltage are applied to the plural signal terminals 2 and 3, the diodes 9 and 10 and transistor 11 do not conduct. In such an input condition, the equivalent circuit in FIG. 10 becomes as shown in FIG. 11. More specifically, the diodes 9, and 10 are replaced by the electrostatic capacities 15 and 16, and the transistor 11 by the parallel circuit of collector resistance 17 and electrostatic capacity 18 between collector and emitter. When such surge protection apparatus is applied to the input part of the circuit processing high frequency signals or switching signals, such as high frequency amplifiers and switching circuits, the electrostatic capacities 15 and 16 are a set at low impedance in the high frequency region of the input signal. Accordingly, the plural signal terminals 2 and 3 are coupled through the low-impedance electrostatic capacities 15 and 16, and the input signal applied to one signal terminal 2 is mixed into the other signal terminal 3, thereby generating a so-called crosstalk. This crosstalk occurs not only in high frequency amplifier and switching circuits but also in high input impedance circuits or high gain circuits even in a low frequency amplifier.
In the surge protection apparatus in FIG. 12, electrostatic capacity is not present between the signal terminal 12 and the other signal terminal. Therefore, in actual use, the crosstalk as experienced in the surge protection apparatus in FIG. 10 does not occur. In the surge protection apparatus in FIG. 12, however, since the junction capacitance of the diodes 13 and 14 is applied between the signal terminal 12 and the pair of power supply terminals 4 and 5, the high frequency characteristic of the electronic function circuit deteriorates.
Generally, moreover, the environments for inducing the electrostatic breakdown of a semiconductor integrated circuit (called an IC hereinafter) incorporating electronic function circuits are:
1. when the IC is put in a magazine and is conveyed, and
2. when the IC is built into an electronic device, among others, and in these circumstances, anyway, friction is applied to the IC from its outside. In such environments, the surge voltage applied to the signal terminal 12 of the IC incorporating the surge protection apparatus in FIG. 12 is discharged through the impedance in the internal circuit 1, or the impedance (in the internal circuit 1, or the impedance) between the pair of power supply terminals 4 and 5. Accordingly, in the surge protection apparatus in FIG. 12, the effect of the surge protection (the discharge capability) varies depending on the magnitude of the impedance between the power supply terminals 4 and 5. For example, in the IC with a small degree of integration, generally, the current consumption is small, and the internal direct-current resistance between the power supply terminals 4 and 5 is large. Furthermore, since the junction area of the resistance element or transistor element formed in one semiconductor substrate is small, the equivalent junction area between the power supply terminals 4 and 5 becomes consequently small. When a positive static electricity is applied to the signal terminal 12 of such an IC with a small degree of integration, the positive charge flows into the internal circuit 1 by way of the diode 13. However since the impedance between the power supply terminals 4 and 5 is large, in other words, the electric charge absorbing capability is weak, the voltage between the power supply terminals 4 and 5 is likely to go up, and a high voltage is maintained for a long period. This tendency appears more significantly when the IC has a large internal impedance between the power supply terminals 4 and 5, and the probability of breaking down the IC by static electricity is raised.
Meanwhile, in the conventional surge protection apparatus shown in FIG. 10, since the discharge route of positive charge is formed by the breakdown voltage BV.sub.CEO of the transistor 11, a favorable surge protective effect is obtained regardless of the impedance between the power supply terminals 14 and 15.
Thus, the two surge protection apparatuses shown in FIG. 10, FIG. 12 have their own merits and demerits. Therefore, the designers of ICs were forced to select the surge protective apparatus estimated to be the best for each IC in consideration of the application of IC and the degree of integration of the circuit elements. The criterion is, at the present, dependent on the intuition of the designers on the basis of the past experience.
In this background there is a growing demand for the development of general-purpose surge protection apparatus applicable to any design specification of IC.
The present invention is intended to present a surge protection apparatus meeting such a demand.